1. Field of the Invention
The present invention relates to an improved electro-optic modulator for reducing the amplitude of crystal resonance. Acoustic energy is efficiently removed from the crystal by the acoustically matched electrodes and side bars where it is dissipated by scattering and absorption.
2. Description of Related Art
All wideband frequency modulated (FM) chirped CO.sub.2 lasers exhibit pronounced amplitude modulation (AM), as a side effect to the desired FM modulation. Excitation of the FM electro-optic modulator crystal's acoustic resonance is one of the drivers of the AM modulation. Reduction of the crystal's resonance amplitude will directly reduce the unwanted AM modulation.
Some solid state lasers having high repetition rate solid state Q-switches also suffer from acoustic oscillation, causing the switch to ring open when it should be shut.
Prior to the present invention, copper was used as the electrode material and alumina (Al.sub.2 O.sub.3) or beryllia (BeO) was used as a side bar material (electrically, it works as an insulator) in many CdTe electro-optic modulators. Indium was also used between the electro-optic modulator crystal and the electrodes to fill up any surface irregularities that improve the acoustic coupling between electrodes and CdTe crystal. These materials do not exhibit acoustic impedances which closely match that of electro-optic modulator's CdTe crystal and therefore contribute to an increase in the amplitude of the electro-optic crystal's acoustic resonance.
Two references that exemplify the use of intracavity CdTe modulators for CO.sub.2 lasers are: R. L. Shoemaker et al, "Frequency-switchable CO.sub.2 laser: design and performance", Applied Optics, Vol. 21, No. 5, pp. 961-966 (1 Mar. 1982) and J. E. Kiefer et al, "Intracavity CdTe Modulators for CO.sub.2 Lasers", IEEE Journal of Quantum Electronics, Vol. QE-8, No. 2, pp. 173-179 (February 1972).
An electro-optic crystal must be damped in some manner to reduce the amplitude of the acoustic resonances. This must be done for a variety of reasons. The crystal can actually fracture under some circumstances if it is not properly damped. Less catastrophic though often vital, the crystal will ring at its natural resonance, giving rise to many problems. These include leakage through an electrooptic shutter, unwanted lasing spikes, frequency chirping, and other effects.
Thus, a need exists for an improved electro-optic modulator that minimizes the above-mentioned disadvantages.